The ballistic performance of thick ultra high molecular weight polyethylene composite

Ultra-high molecular weight polyethylene (UHMW-PE) fibre-reinforced composite is a promising material for ballistic protection due to its high strength, stiffness and low density. For thick sections of this material however, there is limited understanding of the mechanisms driving ballistic performance. Existing analysis tools do not allow for a good approximation of performance, while existing numerical models are either incapable of accurately capturing the response of thick UHMW-PE composite to ballistic impact or are unsuited to model thick targets.

In this thesis, the response of thick UHMW-PE composite to ballistic impact was experimentally investigated. Panel thicknesses ranging from 9 mm to 100 mm impacted by 12.7 mm and 20 mm calibre fragment simulating projectiles (FSPs) were investigated. The penetration and failure mechanisms were identified by inspection of impacted targets, and scanning electron microscopy was conducted to inspect the load-bearing fibres around the penetration cavity. Thick targets demonstrated a two-stage penetration process: shear plugging during the initial penetration followed by the formation of a transition plane and bulging of a separated rear panel.

A new analytical model was developed for thick UHMW-PE composite impacted by blunt projectiles to describe the two stages of penetration identified in the experimental work. The analytical model is based on energy and momentum conservation laws, and uses an energy balance between the projectile kinetic energy and the energy absorbed by the target to predict the ballistic limit velocity. The model was validated against experimental ballistic limit results and showed excellent agreement for thick targets. Existing analytical models based on membrane theory were validated against results of thinner targets and demonstrated to be more suitable.

A numerical modelling methodology was developed for the ballistic impact analysis of thick UHMW-PE composite using a commercial hydrocode. A novel approach was used to model interlaminar failure by dividing the panel into sub-laminates connected by breakable bonds. A new failure-based element erosion model was implemented with a user subroutine, which more accurately accounts for the directional properties of fibre-reinforced composites than existing strain-based models. The model is extensively validated against experimental ballistic data. The model gave excellent predictions for depth of penetration, residual velocity and ballistic limit, with results within 5% of experiment for all conditions considered (12.7 mm and 20 mm FSP, 9 mm to 100 mm thick targets and impact velocities between 400 m/s to 2000 m/s). The predictions of the penetration mechanisms and target bulge behaviour were also compared in terms of the target shear plugging ratio and bulge hinge and apex position, also demonstrating very good correlation with the experimental results.